1. Field of the Invention
The present invention relates to semiconductor devices, and more particularly to a semiconductor device having a semiconductor chip provided on a circuit substrate.
2. Description of the Background Art
In the field of semiconductor devices, hopes for power devices that can contribute to enhanced energy efficiency have risen for achieving low carbon society. Semiconductor chips used in the field of power devices have been more integrated and have operated at higher speeds. Consequently, the semiconductor chips have had higher power densities and higher heat generation. In response to this, the semiconductor devices have been required to have a higher capability of heat radiation. A capability of heat radiation considerably depends on circuit substrates where the semiconductor chips are mounted.
Japanese Patent Application Laid-Open Publication No. 2007-134563 discloses a circuit substrate including a ceramic substrate and a metal circuit pattern formed by etching. The ceramic substrate has a metal layer bonded on its back surface. The metal layer is used to bond a radiator plate onto the layer. Moreover, the publication indicates that a thick part and thin part of a metal circuit need to be formed on one substrate in view of the fact that a power part and a control part of a power module have different amounts of current flowing through an element.
Japanese Patent Application Laid-Open Publication No. 2007-201346 indicates a problem when a metallized layer (metal layer) and a heat spreader (radiator plate) are bonded by soldering. Specifically, the publication indicates that presence of a solder layer causes the heat spreader to have an insufficient capability of heat radiation. Accordingly, in a technique described in the publication, a ceramic substrate has a thick-film heat-radiating conductor circuit formed on its surface, where the circuit is formed of a sintered body of metal particles.
In a circuit substrate, a circuit pattern and an insulating plate that supports the pattern are made of different materials. If temperature of a semiconductor device changes, warpage occurs in the circuit substrate due to a difference of thermal expansion coefficient between the different materials. Techniques described in the publications described above fail to consider preventing such warpage.